Shrouded impellers are used routinely in certain turbomachines such as centrifugal pumps, compressors, and in high efficiency turbines, such as, for example, in turboexpanders used to produce refrigeration by expansion of the process gas in cryogenic gas separation, refrigeration or liquefaction cycles. Since the fluid pressure is higher at the outer diameter of the impeller as compared to the pressure at the inner diameter of the impeller at the impeller eye, a non-contacting seal, such as a labyrinth seal, is customarily used to reduce the bypass or recirculation of the working fluid lost between the stationary walls of the turbomachine housing and the impeller shroud. This bypass or recirculation fluid loss is wasteful and an attempt is usually made to minimize this loss by designing tighter fitting seals with an increased number of sealing lips. Unfortunately, this approach is limited by two effects. First, tight and long seals tend to impose a cross coupling force on the bearings resulting in a destabilizing effect and, second, the friction forces will increase in a tight and long seal to a value where they could overwhelm the recirculation or bypass losses.
Whether the machine is a turbine or a compressor handling gaseous compressible fluid, or a pump handling liquid, the pressure at the outer diameter of the impeller is greater than that at the inner diameter. Thus, the higher pressure at the impeller outer diameter will cause part of the working fluid to bypass the wheel in case of the turbine or set up a recirculation flow in the case of a compressor or pump. It can be appreciated that this bypass or recirculation flow represents an undesirable parasitic loss.
Generally, there are three loss mechanisms involved. The first, in the case of turbines, is due to the fact that the portion of working fluid which bypasses the wheel does not perform external work but rather undergoes a Joule-Thomson expansion. Contrary to this lack of external work for a turbine, in a compressor or pump, external work has to be performed repeatedly on the recirculating portion of the working fluid.
Another type of loss mechanism generated by the bypass or recirculation flow is due to the aerodynamic behavior of the flow in diffusers. Whether the turbomachine is a turbine, compressor or pump, the fluid will have the lowest static pressure at or around the impeller inner diameter. Thus, part of the fluid velocity head will be converted with a certain efficiency to pressure downstream of the impeller eye. Injecting the bypass or recirculation flow at the inlet end of the impeller is deleterious due to increasing the thickness of the boundary layer. This reduces the efficiency of the pressure recovery by causing the boundary layer to separate from the turbomachine walls. Even when it is carefully optimized by use of efficient seals, the recirculation or bypass fluid flow is on the order of one percent of the working fluid flow. While this may not appear to be excessive, unfortunately this fluid is injected into the main flow at a very unfavorable location i.e. at a point after which deceleration of the main flow relative to the surrounding walls occurs.
The third loss mechanism is due to the fact that the temperature of the bypassed or recirculating fluid is higher than that of the turbine outlet or compressor and pump inlet at the impeller inner diameter. Therefore, the compressor or pump will have to work against a higher average temperature resulting in yet higher work input. In the case of a cryogenic turbine operating for example in a liquefaction cycle, the heat will be added at a low temperature point of the cycle and subsequently must be heat pumped and discharged at ambient temperature level.
Accordingly it is an object of this invention to provide an improved turbomachine wherein the inefficiency caused by the flow of recirculation or bypass fluid is reduced.
It is another object of this invention to provide an improved method for operating a turbomachine wherein the inefficiency caused by the flow of recirculation or bypass fluid is reduced.